Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to reducing reference signal overhead in wireless communication systems.
Background
Wireless communication networks are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, and the like. These wireless networks may be multiple-access networks capable of supporting multiple users by sharing the available network resources. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the Universal Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). Examples of multiple-access network formats include Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA) networks.
A wireless communication network may include a number of base stations or node Bs that can support communication for a number of user equipments (UEs). A UE may communicate with a base station via downlink (DL) and uplink (UL). The downlink (or forward link) refers to the communication link from the base station to the UE, and the uplink (or reverse link) refers to the communication link from the UE to the base station.
A base station may transmit data and control information on the downlink to a UE and/or may receive data and control information on the uplink from the UE. On the downlink, a transmission from the base station may encounter interference due to transmissions from neighbor base stations or from other wireless radio frequency (RF) transmitters. On the uplink, a transmission from the UE may encounter interference from uplink transmissions of other UEs communicating with the neighbor base stations or from other wireless RF transmitters. This interference may degrade performance on both the downlink and uplink.
Release 10 of 3GPP Long Term Evolution (LTE), for example, provides for 9 downlink transmission modes. These modes include transmit diversity, Multiple-Input/Multiple-Output (MIMO), Code Division Multiplexing (CDM), etc. Downlink transmission mode 9 supports up to MIMO rank 8 Demodulation Reference Signal (DM-RS) based Physical Downlink Shared Channels (PDSCHs).
The DM-RS based PDSCH uses a predetermined DM-RS pattern, examples of which are shown in subframes 100A (normal subframe with 14 symbols), 100B (Downlink Pilot Time Slot (DwPTS) subframe with 11-12 symbols), and 100C (DwPTS subframe with 9-10 symbols) of FIGS. 1A-1C, in all of the Resource Blocks (RBs) assigned to the PDSCH to facilitate decoding of the PDSCH. For example, for PDSCHs transmitted at MIMO rank 2 or below, DM-RS patterns comprising 12 Resource Elements (REs) may be used, as represented by either the 12 dark shaded REs (REs 111) or the 12 light shaded REs (REs 112) of FIGS. 1A-1C. The particular DM-RS patterns used may be optimized for specific scenarios (e.g., the DM-RS pattern provided by REs 111 may be optimized for a first CDM group while the DM-RS pattern provided by REs 112 may be optimized for a second CDM group). For PDSCHs transmitted at MIMO ranks above 2, DM-RS patterns comprising 24 REs may be used, as represented by the 24 REs of both the dark and light shaded REs (REs 111 and 112). It should be appreciated that other resource elements of these frames may be occupied by other signals, such as Common Reference Signals (CRSS), also referred as cell-specific reference signals, as represented by the cross-hatched REs in FIGS. 1A-1C.
In addition to the different patterns and number of REs that may be utilized, DM-RS based PDSCH may further implement different spreading factors (SFs). For example, for PDSCHs transmitted at MIMO rank 4 or below, a spreading factor of 2 (SF2), whereby the DM-RS pattern is spread across 2 consecutive Orthogonal Frequency Division Multiplexing (OFDM) symbols in time, may be used. Whereas, for PDSCHs transmitted at MIMO ranks greater than 4, a spreading factor of 4 (SF4), whereby the DM-RS pattern is spread across 4 OFDM symbols in time in a subframe, may be used.
As can be appreciated from the foregoing, various aspects of the demodulation reference signal are optimized for use with respect to particular communication scenarios. For example, the DM-RS pattern, number or REs utilized, and the spreading factor implemented, may be varied depending upon such considerations as the MIMO rank, the CMD groupings, etc. Depending upon the particular demodulation reference signal configuration utilized, an appreciable amount of the otherwise available communication bandwidth may be consumed with this reference signal overhead.
As the demand for mobile broadband access continues to increase, the possibilities of interference and congested networks grows with more UEs accessing the long-range wireless communication networks and more short-range wireless systems being deployed in communities. Research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications.
For example, LTE releases 8-10 provide for utilization of 1 of 7 possible time division duplex (TDD) downlink/uplink subframe configurations, as shown in the table below wherein a subframe designated with a D is a downlink subframe, a subframe designated with a U is an uplink subframe, and a subframe designated with a S is a special subframe (special subframes having a downlink portion for control, and possibly data, and an uplink portion for channel sounding, and possibly random access preamble transmission). Selection of a particular subframe configuration in accordance with LTE releases 8-10 is relatively static, and thus tends to remain the same once selected for hours and even days. However, there is ongoing discussion with respect to the development of LTE release 11 to potentially provide for dynamically adapting the TDD downlink/uplink subframe configurations.
Uplink-downlink configurations.Downlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD6 5 msDSUUUDSUUD
In developing LTE release 11, the possibility of providing dynamic selection of the TDD downlink/uplink subframe configuration based upon actual traffic needs is being explored. For example, if during a short duration a large data burst on downlink is needed, the TDD downlink/uplink configuration may be changed from a relatively symmetric downlink/uplink configuration (e.g., configuration 1 providing 6 DL slots and 4 UL slots) to a downlink/uplink configuration favoring the downlink (e.g., configuration 5 providing 9 DL slots and 1 UL slot). Such dynamic adaptation of the downlink/uplink configurations may, however, result in increased interference to both downlink and uplink, such as when two or more cells have different downlink and uplink subframe configurations. Moreover, the changes in downlink/uplink configuration, and thus the interference introduced thereby, may occur rapidly. For example, the adaptation of the TDD downlink/uplink configuration is expected to be no slower than 640 ms, but may occur as quickly as 10 ms.
With the demand for mobile broadband access continuing to increase, it is expected that future wireless networks will comprise more densely deployed nodes (e.g., basestations, access points, etc.). Such wireless network nodes may include high power class nodes, such as to provide wireless communication coverage of relatively large areas, and low power class nodes, such as to provide throughput reachability and/or throughput enhancement. For example, low power class nodes may be deployed in the wireless network to provide increased throughput in a heavy traffic area, to enable high bandwidth wireless communications in an area unserved by high power class nodes, etc.
The low power class nodes of a wireless network may comprise planned and/or unplanned deployments, may comprise fixed locations and/or dynamic locations (e.g., UE relays), etc. Nevertheless, it is expected that the low power class nodes will serve a limited number of UEs, in contrast to traditional macro nodes (high power class nodes) where each node typically serves a larger number of UEs. For example, in an extreme case, a low power class node may serve a single UE for some duration.
Such low power class nodes may provide deployments which are more near the served UEs. Moreover, due to their serving fewer UEs, the number of activities being scheduled by these particular nodes is limited. Accordingly, it is expected that the channel condition between the low power class node and the served UEs will be more favorable, as compared to the channel condition between typical high power class nodes and their served UEs. For example, the channel condition between a low power class node and its served UEs may be slow time-varying, have a small multi-path delay profile, provide a high signal to noise ratio (SNR), etc.
It should be appreciated that the aforementioned DM-RS patterns generally in use today are adapted for use with respect to the typical high power class nodes. Due to more favorable channel conditions expected to be associated with the low power class nodes, as well as the expectation of very limited number of UEs served by a node, the above DM-RS patterns are not optimized for use with respect to many instances of low power class nodes. A possibility of reducing control and reference signal overhead is therefore presented. If achieved, a reduction in control and reference signal overhead may facilitate the allocation of more resources to actual data transmissions in the wireless network. However, techniques for reducing control and reference signal overhead should balance the reduction in overhead with any resulting impact on data channel decoding, such that overall system performance gain can be achieved. In particular, a balance between the reference signal used and the corresponding data to be demodulated using the reference signal should be struck for proper estimation of the condition of the channel, channel information, timing information, etc. in order to reliably demodulate the data.
Although the opportunity exists for reducing reference signal overhead in the more favorable channel conditions expected with low power class nodes, the channels associated with these nodes may nevertheless be slow time-varying. Accordingly, the use of reference signal overhead reduction techniques may result in some undesired impact on data decoding. Moreover, although a channel may be slow time-varying, interference may vary considerably from subframe to subframe, particularly if the dynamic selection of the TDD downlink/uplink subframe configurations proposed with respect to LTE release 11 are implemented. Therefore, some techniques for reducing reference signal overhead may not be acceptable in the interference environment presented. Also the impact on Hybrid Automatic Repeat Request (HARQ) operation, whether known or unknown, by a reference signal overhead reduction technique may render the technique undesirable.